Measurements and Modeling of Granular Flows in the Collisional Lubrication Regime

Abstract

This paper concerns an annular flow sliding shear cell experimental apparatus in which we have observed a process we call granular collisional lubrication. There is a critical speed when the motion becomes smooth and the surface separation increases. Presumably, the particles are strongly agitated by the sliding surface and colliding with one another, rather than rubbing. This is the onset of granular collision lubrication. Among the key experimental findings are that for both sloping and parallel surfaces the normal (pressure) and shear stress are proportional to the square of the shear rate. The primary feature of granular collisional lubrication is that significant load carrying forces are generated from the so-called “wedge effect” as in hydrodynamic lubrication, but equally from the collisions in simple parallel shear flow (the flat surface case). We present a constitutive equation which can exhibit a rheological normal stress in parallel flow and thus better model granular lubrication as a quasi-hydrodynamic process.